The present invention relates generally to automotive HVAC systems and methods of operating such HVAC systems.
Automotive HVAC systems are well known and are utilized for heating and cooling the passenger compartments of vehicles. Hybrid vehicles, which utilize a battery and an intermittently operated internal combustion engine for vehicle propulsion, have difficulty keeping the passenger compartment cool when the engine is off. When the engine is off, the HVAC compressor, typically run by a clutch connected to the engine, is also off and the temperature in the passenger compartment can rise quickly. Since the majority of prior art automotive HVAC compressors are mechanically coupled with the internal combustion engine through an accessory system such as a clutch or the like, turning off the engine suggests a deteriorated occupant comfort due to the fact that the refrigerant compressor is non-operational while the engine is off. To maintain a certain level of air conditioning performance, the engine must be restarted, which has then a negative impact on the fuel economy of the hybrid vehicle.
A common prior art automotive HVAC control system utilizes temperature-based control wherein a temperature sensor monitors ambient temperature and sends electrical signal(s) to a HVAC control module. A control algorithm embedded into the control module compares the temperature reading with an established temperature criterion. Based upon the algorithm, a control action will be executed to either couple the compressor to or decouple it from the accessory drive by engaging or disengaging the compressor clutch.
Basing the control of the HVAC system on air temperatures alone has raised concerns in recent development of mild hybrid vehicles. One concern includes deteriorated air conditioning performance in high humidity and medium temperature ambient conditions due to the tendency to cause a musty odor to emanate from the air conditioning outlets when the engine is off. Another concern is an excessively negative impact on fuel economy in low humidity and medium to high temperature, low temperature and high humidity, and medium temperature and low to medium humidity ambient conditions because the temperature-based control method keeps the engine running or starts the engine earlier than is desirable. Fundamentally, temperature-based control is a single variable control system, in which temperature is disadvantageously the sole variable used for making operational decisions. This disadvantage is particularly undesirable when such temperature-based control is applied with a hybrid vehicle.
It is desirable, therefore, to provide a more efficient control methodology for operating an automotive HVAC system, including hybrid vehicles.